1. Field of Invention
The present invention relates to ceramic materials where the materials are ceramic matrix composites reinforced by continuous-length small-diameter ceramic fibers that are formed in complex shaped architectures by conventional textile processes. In particular, the present invention is based on improving the damage tolerance and oxidative durability of ceramic matrix composites consisting of non-oxide or oxide ceramic matrices reinforced by ceramic fibers with non-oxide chemical compositions.
2. Description of Related Art
As power and propulsion systems advance, there are many applications that require new materials that are expected to withstand high stresses for extended times at higher temperatures than state-of-the-art metallic alloys (above ˜1100° C.). Such requirements generally arise in commercial, industrial, and military areas requiring improved engines for transportation, energy production, and energy conversion. This is particularly true for aero-based and land-based gas turbine engines, where improving efficiency and reducing emissions is accomplished by raising the temperature capability of the material in hot section components like combustors, vanes, and blades that must also function under high mechanical, thermal, and aerodynamic stresses. Most of these high temperature applications also have oxidative service conditions due to the combustion of fuel and oxygen and to the cooling of components by ambient air. Oxidative conditions can also occur in space-based engines where one burns hydrogen or jet fuel in oxygen obtained from the liquid state.
Ceramic composites with oxidation-resistant ceramic matrices reinforced by complex architectures of continuous-length fibers with non-oxide chemical compositions, such as silicon carbide (SiC), have many of the desired properties for these applications and compete against monolithic ceramic materials with similar compositions as the matrices. However, the monolithic materials often fracture catastrophically and so fiber-reinforced composite materials are generally required for graceful failure and significantly improved toughness and damage tolerance. This enhancement arises from the deposition of a thin mechanically weak coating or interphase on the fibers to cause random cracks in the ceramic matrix to be deflected around the fibers, thereby allowing the fibers to remain intact, to bridge the matrix cracks, and to carry the structural loads applied to the composite. Although other fiber types based on high-temperature oxide compositions can be used to bridge cracks, at temperatures above 1100° C. current state-of-the-art oxide fibers creep and rupture at significantly lower stresses than non-oxide fibers with such base compositions as SiC, silicon nitride (Si3N4), and carbon. Thus non-oxide fiber-reinforced ceramic matrix composites, such as SiC fiber-reinforced SiC matrix composites (SiC/SiC), are currently gaining the most technical interest for replacing metallic alloys for the hot-section components of advanced power and propulsion systems. Because of the complex shapes of these components and the need for reinforcing fibers in multi directions, conventional textile processes such as weaving and braiding are generally required in order to form net-shape fiber-architecture preforms that are eventually infiltrated with the ceramic matrix material.
One potential issue with non-oxide fibers is that if the ceramic matrix composite is cracked by unforeseen stresses, the crack-bridging fibers and their interphases will be exposed to oxidizing environments that enter the composite through matrix cracks that terminate at the composite surface. If the matrix cracks are also allowed to reach the fiber surfaces, the oxygen will attack the non-oxide fibers by forming volatile oxides or forming performance-degrading oxide layers on the fiber surface. Ceramic materials with Si-based compositions are the most resistant to oxygen in that their oxide layers with compositions based on silicon dioxide (i.e., silica) are the slowest to grow of any non-oxide ceramic. Thus, for long-term service of cracked composites, fibers based on SiC and Si3N4 are the most preferred of all the non-oxide fibers. However, although silica growth is slow, even a small amount can cause contacting fibers to bond to each other and to Si-based matrices, thereby eliminating the ability of each fiber to act independently. The detrimental consequence is that if one fiber should fracture prematurely, all others to which it is bonded will fracture, causing catastrophic composite fracture or rupture at low stresses and short times. This oxidation issue for Si-based fibers typically begins at intermediate temperatures (600 to 800° C.) where the silica formation is slow but sufficient to cause fiber-fiber and fiber-matrix bonding.
To minimize the intermediate temperature oxidation problem for composites reinforced by non-oxide fibers, prior art contains a variety of approaches to prevent the cracks from reaching the fiber surfaces (e.g., patents: Goujard et al., U.S. Pat. No. 5,738,951; Fareed et al., U.S. Pat. No. 6,228,453 and in the literature: H. W. Carpenter and J. W. Bohlen, Ceramic Engineering and Science Proceedings, vol. 13, no. 7-8, pp. 23-36 (1992)). For these approaches, the general objective is to design an interphase structure so that prior to the crack reaching the fiber, local mechanical contact between the fiber and matrix is lost either within the interphase structure or on the outside of the interphase (outside debonding). The remaining interphase material on the fiber surface will then slow down silica formation on the fiber, provided the interphase composition can provide some oxidative stability. A typical approach of prior art is to deposit multi-layer interphases on the fibers that consist of thin oxidation-resistant layers like SiC separated by thin and weak crack deflection layers like carbon, boron nitride, or porous oxides (U.S. Pat. Nos. 5,738,951 & 6,228,453). However, all prior art patents related to the use of the outside debonding mechanism are based on the interphase debonding from the matrix to occur during matrix crack propagation, thus requiring similar microstructure conditions to exist locally near every interphase, even for complex fiber architectures produced by conventional textile processes.